1. Field of the Invention
The present invention relates to a tuner unit for communications apparatus and in particular to television and radio receivers employing a phase locked looped system which upon tuning of the receiver apparatus successively tunes the individual tuning circuits to the frequency generated by the phase locked loop and which retains the tuning voltages of the individual filter circuits obtained in this manner.
2. Brief Description of the Background of the Invention Including Prior Art
The German Patent Application Laid Open DE-OS 28 54 852 teaches such a tuning unit for example, which tunes the high frequency circuits to the desired receiver frequency with three auxiliary oscillators and corresponding analog storage units using a phase locked loop circuit (PLL-circuit). Auxiliary oscillators are required corresponding to the number of high frequency circuits to be balanced. The number of the auxiliary oscillators required and thus the material expenditures are increased particularly if in case of a television receiver the band I, band III and the bands IV and V are to be balanced. There exists the danger of ambiguities between the frequencies of tuner oscillator, of the auxiliary oscillators and of the receiver frequencies. It is disadvantageous for a practical realization of the known tuning units that the exciting windings of the individual auxiliary oscillators can cause undesired resonances and damping of the circuits in a reception case, which can also cause an uncontrolled detuning of the circuits, if the auxiliary oscillators are to be switched off after performed balancing.
A tuning method for radio receivers is taught in the journal "Nachrichten elektronik" Issue 11/79, pages 365 to 368, which also employs additional auxiliary circuits in the high frequency filters. Here again disadvantageous interferences of the circuits result during operation of the balanced receiver. As indicated on page 367 in the left and middle column, the precision of the tuning is achieved only based on exactly paired tuning diodes. This again requires an effort which cannot be neglected, since the tuning diodes have to be realized on the same chip.